AC Milan's Dutch midfielder Seedorf was the target of racist chants from Lazio fans in a 1-1 draw in Serie A, which led to the Rome club being given a 15,000 euro fine by the Italian Football League on Monday.
Zebina, meanwhile, blamed racism after being slapped in the back of his neck by a supporter of his troubled Juventus side after a 2-1 win over Atalanta.
"The Italian Footballers' Association expresses total solidarity and support for Jonathan Zebina and Clarence Seedorf, who were hit by despicable incidents of racism on Sunday," the body representing Italy's professional players said on Monday.
"In firmly condemning these shameful acts, the AIC hopes that all elements in football will work with concrete actions to combat these expressions of incivility by a violent minority that must be isolated and silenced at all costs."
French defender Zebina also said he was provoked by racists when he showed his middle finger to his side's supporters during a 4-1 Europa League defeat at Fulham earlier this month.
The Italian Football League said on Monday that it had fined second-placed AS Roma 25,000 euros for a racist chant and other disturbances by their fans during Saturday's 2-1 win over leaders Inter Milan.
Italian football has a big racism problem and Juventus had to play a game behind closed doors last season because of offensive chants by their fans.
